Method of tape bonding

ABSTRACT

A tape bonding system may include a tape and a lead coupled to the tape. The lead may have a stress relief formed along its length which is adapted to relieve stresses and strains arising from differential thermal expansion of the materials to which the tape is coupled. The lead may have a first portion supported by the tape and a second portion which is unsupported by the tape due to an opening in the tape. The unsupported portion of the tape may be deflected to make contact with a bond pad. In this configuration, the lead may accommodate differential thermal expansion by cantilevered beam displacements. In addition, due to the provision of a stress relief, the beam can respond in a totally different rotational displacement to compressive stresses parallel to the surfaces of the integrated circuit elements being joined.

This is a divisional of prior application Ser. No. 09/376,699 filed Aug.18, 1999 now U.S. Pat. No. 6,362,429.

BACKGROUND

This invention relates generally to tape automated bonding techniquesfor making electrical connections to integrated circuit devices.

Integrated circuit devices in the form of an unpackaged die or chip aretypically packaged to provide electrical connections. These electricalconnections allow the die or chip to be connected to other devices.While it is possible to produce very small elements in integratedcircuit devices, because of the mechanical nature of the contacts,generally the contacts must be large relative to the size of the die orchip.

Therefore, it is not uncommon that the contacts on an integrated circuitpackage take as much surface area as the entire integrated circuit dieitself. However, there is a continuing effort to reduce the size andspacing between such contacts to allow ever smaller integrated circuitpackages. Such interconnection assemblies in relatively small dimensionsare commonly called fine pitch assemblies.

One popular interconnection technique is a so called ball grid array(BGA). An array of solder balls may be coupled to another component bysimply contacting bonding pads on one device to the solder balls onanother device and heat reflowing to activate the solder. Techniques forproviding very fine pitch ball grid array packages are commonly calledfine pitch ball grid array (FBGA) packaging.

Thus, in one example, an integrated circuit device having a plurality ofsolder balls extending outwardly from the package may simply be placedon a printed circuit board. When subjected to temperature reflow, theintegrated circuit device is automatically connected to the printedcircuit board in a so-called surface mount technique.

Tape automated bonding or TAB provides a high speed technique forproviding interconnections to chips. In effect, an interconnection layerprovided in the form of a continuous adhesive tape may be secured to thedie in an automated fashion. Metallic leads inside the tape layer maythen be caused to contact bond pads on the die. At the same time, theother side of the tape layer may be adapted to make connections to theoutside world. For example, tape layers may be coupled to solder balls.In this case, the tape may make electrical connection to bond pads onthe die on one side and may couple to solder balls on the other side forconnection to the outside world. A layer of traces within the tape layermay be used to connect the solder balls to the bond pads on the die.

Because of the different materials involved in making the die and thetape layers, the tape layers may have a different temperaturecoefficient of expansion (TCE) than the die or chip to which it isconnected. This may result in failure of the interconnection andultimately the loss of the entire packaged integrated circuit device.

Techniques are known for facilitating the relative expansion between thetape layer and the die. For example, in one known technique, the tapelayer includes a cantilevered metallic beam which is deflected to makecontact with bond pads on the die. Through an elaborate procedure, thebeam is bent down to make contact with the die in a way which, ineffect, provides a bow in the beam. This is done by pushing the beamdown and axially towards its support at the same time, creating thebowed shape. In this way, the relative thermal expansion may beaccounted for by bending motion within the bowed cantilevered beam.

As an analogy, the cantilevered beam is attached on two ends like a bowfor a bow and arrow as indicated in FIGS. 6A and 6B. When the ends movetowards each other as indicated by the arrows I and J in FIG. 6A, thebeam simply deflects outwardly, as indicated by the arrow K, on theconvex side of the beam to accommodate for the relative motion.Similarly, in response to a vertical displacement, indicated by thearrows L and M in FIG. 6B, the beam bows in the direction of the arrowN. Displacement of the ends of the beam, in substantially any direction,is transformed into either an increase or decrease in the bowing of thebeam around the same central axis, indicated at C, of the beam.

This bowing has a beneficial effect in one sense because the stress isadvantageously relieved, increasing the life of the system. However,regardless of the direction of the applied force, the response of thebeam is the same. It always deflects around the same axes. This resultsin an increase in the amount of strain which must be withstood by thebeam. In addition, the bowing tends to concentrate stresses at thepoints where the ends of the beam are connected to the die or the tape,thereby creating a stress riser at these locations. Unfortunately, theselocations are among the most highly stressed in the entire system,increasing the possibility of failure at these locations.

Thus, there is a continuing need for better ways to relieve stress intape bonding systems and especially for better ways to relieve stress infine pitch assemblies.

SUMMARY

In accordance with one aspect, a tape bonding system includes a tape anda conductive lead. The lead is situated in a surface and is secured tothe tape. The lead is adapted to be bonded to a bond pad at one locationand to be supported by the tape at another location. A stress relief isformed in the lead between the two locations. The stress relief isadapted to convert stress along the lead into rotation about an axissubstantially transverse to the surface in which the lead is situated.

Other aspects are described in the accompanying detailed description andclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged cross-sectional view of one embodiment of thepresent invention;

FIG. 2 is an enlarged view of a portion of the embodiment shown in FIG.1;

FIG. 3 is a partial, enlarged inverted view of a portion of theembodiment shown in FIG. 1 before the lead is deflected to contact adie;

FIG. 4 is a greatly enlarged bottom plan view of the embodiment shown inFIG. 1 with the solder balls removed;

FIG. 5A is a schematic bottom plan depiction of the embodiment shown inFIG. 4 illustrating the response of the system to differential thermalexpansion;

FIG. 5B is a schematic side elevational depiction corresponding to theview shown in FIG. 3 showing the response of the system of differentialthermal expansion;

FIG. 6A is a side view corresponding to FIG. 5B in a prior artembodiment, illustrating the response to compressive stress;

FIG. 6B is a side view of a prior art lead illustrating its response toa compressive stress applied transversely to the direction illustratedin FIG. 6A;

FIG. 7 is an enlarged bottom plan view of still another embodiment ofthe present invention;

FIG. 8 is an enlarged bottom plan view of still another embodiment ofthe present invention;

FIG. 9 is an enlarged bottom plan view of yet another embodiment of thepresent invention; and

FIG. 10 is an enlarged bottom plan view of still another embodiment ofthe present invention.

DETAILED DESCRIPTION

A tape bonding system 10, shown in FIG. 1, may include a conductive lead20 sandwiched between a pair of dielectric layers 12 and 14. The layer14 may be an elastomeric layer which may be formed, for example, ofsolder mask material. The layer 12 may be formed, for example, of arelatively resilient polyimide layer. A plurality of openings 16 may beformed in the dielectric layers 12 and 14, leaving the lead 20 generallyunsupported at those locations by the tape bonding system 10. Inaddition, a plurality of openings in the dielectric layer 14 may befilled by solder balls 18.

The tape bonding system 10 may be connected to an integrated circuit dieor chip 21 by adhesive securement techniques. The die 21 may includebond pads 22 which may be arranged adjacent the openings 16 through thedielectric layers 12 and 14. As illustrated, the conductive lead 20 isdeflected in a U-shaped configuration to make electrical contact to abond pad 22. Similarly, each lead 20 is electrically coupled to a solderball 18 as well.

Thus, in the illustrated embodiment, a pair of leads 20 a and 20 bhaving a gap 15 between them are situated within the same tape systemfor one die 21. The lead 20 a is connected to a bond pad 22 a and thelead 20 b is connected to a bond pad 22 b. However, in some embodiments,only a single lead may be provided across the die. Conventionallyhowever, a number of leads may be provided in the tape bonding system 10displaced in and out of the plane shown in FIG. 1. Thus, an electricalcoupling may be achieved between the die 21 and the solder balls 18 byway of the leads 20.

As shown in FIG. 2, the U-shaped portion 24 of the lead 20 a in theopening 16 is deflected towards the die 21. This deflection is possiblebecause of the support, at either side of the opening 16, provided theelastomeric dielectric layer 12.

As shown in FIG. 3, the connection to a bond pad 22 may be supplied bydeflecting the lead 20 in the Z-axis direction, to assume the dashedline position, causing bends to occur at 26. When the lead portion 30contacts the bond pad 22, the lead may be bonded thereto using heat,sonic energy or other conventional bonding techniques. In thisconfiguration, best shown in FIG. 3, effectively a leaf spring existsbetween the portion 30 and the supported portions 32 and 34 of the lead20.

Thus, expansion and contraction of the die 21 relative to the tapebonding system 10 may result in bending at the bends 26 which amounts tosubstantially a spring stress relieving system. In such case,differential thermal expansion may be accommodated in the Z axisdirection. In other words, if the die 21 and the tape bonding system 10expand or contract differentially because of their different temperaturecoefficients of expansion (TCE), this may be accommodated for throughbending at the bends 26.

Referring to FIG. 4, the overall shape of the lead 20 in an opening 16is illustrated. The lead 20 includes a pair of U-shaped stress relievingportions 32 and 34 on either side of the portion 30 secured to the bondpad 22. In addition, an anchor tab 36 is provided to anchor the lead 20on one side of the opening 16 and a supported portion 33 may be providedon the other side of the opening 16. The S-shaped lead portion proximateto the portion 30 is useful in providing greater tolerance in aligningthe portion 30 to the bond pad 22. If there is slight misalignment, theadjacent portions of the stress relieving portions 32 or 34, indicatedat 36 and 38, may still align with the bond pad 22 providing a usefuldevice.

The response of the system 10 to differential thermal expansion is bestshown in FIGS. 5A and 5B. FIG. 5B shows the response to differentialthermal expansion/contraction in the directions of the arrows E and G.In FIG. 5B the arrows E correspond to displacement of the tape system 10in response to thermal expansion of the tape system relative to a deviceto which the tape system is electrically coupled. The arrows G indicatethe displacement of the die 21. Differential thermal expansion resultsin the motion H1 and H2 in the two sections 32 and 34. Thus, relativethermal expansion in the vertical direction in FIG. 5B may beaccommodated for by displacements of the lead 20 in the direction of thearrows H1 and H2. Such displacements are effectively rotations about theaxis substantially within the surface that the lead 20 occupies.

Conversely, referring to FIG. 5A, differential thermal expansionindicated by arrows A and C results in a different stress reliefmechanism. In this case, stress relieving portions 32 and 34 are causedto deflect around the axes B. Thus, in response to differential thermalexpansion or contraction, the leads bow as indicated in FIG. 5B. Inresponse to differential thermal stresses in the transverse direction,the lead portions on either side of the axis B respond around the axis Btraverse to the plane of the lead 20. In this way, there are effectivelytwo different responses to two different stresses. This effectivelyincreases the life of the leads.

Particularly, the stresses illustrated in FIG. 5A are believed to be themost important and most likely stresses. It can be seen that theresponse of the system is to rotate about the axis B. This removes thestresses from the areas 40 and 42 which are believed to be, in mostcases, the most sensitive areas of the entire system. Because of thedeflections already induced at these areas, these areas may be the mostprone to failure. Therefore, by providing the stress relievers 32 and34, the maximum stress and strain have been moved from sensitive areasto areas which have been specifically designed to accommodate suchstresses and strains.

In some embodiments, to further ensure that these stresses are displacedfrom the sensitive areas, the areas proximate to the rotational axes Bmay be weakened or reduced in thickness. As a result, these weakened orreduced areas preferentially absorb such stresses and strains.

It is believed that the strains shown in FIG. 5A are likely to be themost common or predominate strains because they correspond to the majorsurfaces of the components being joined. It is believed that thepredominant thermal expansion will be in these directions because of thegreater dimensions, in these directions, of the objects being joined.

To further illustrate the point, FIGS. 6A and 6B show the response of asystem which does not include the stress relievers 32 and 34. In FIG. 6Ain response to a compressive stress, it can be seen that the lead actsas effectively a bow, bowing outwardly in the direction of the arrow K.This is also an effective rotation in the direction of the arrows aboutthe axis C. Similarly, in response to a vertical compressivedisplacement, effectively the same response occurs. Again, the systemresponds the same way to forces in different directions increasing theconcentration of strain which a location encounters over its life.

Moreover, the system tends to accentuate the strains which must be bornenear the points of attachment indicated by Xs in FIGS. 6A and 6B. Theseareas, which have already been subject to bending to form the lead, tendto be the most sensitive if not the most strain intolerant regions ofthe lead system. Thus, the provision of the stress relievers 32 and 34improves the performance of the system in response to the predominantstrain likely to be encountered and provides an alternate strain reliefmechanism which may increase the lifetime of the system 10 in someembodiments.

Referring to FIG. 7, another embodiment of the present invention isillustrated. In this case, a bond pad 22 is coupled by an L-shaped lead70 to a support structure 72. However in response to the predominantforces of the type indicated in FIG. 5A, the bend 74 acts as a stressreliever. The bend 74 causes rotation about an axis transverse to thesurface containing the leads 70. Again a differential response tostrains in different directions is created and also the response of thesystem is moved away from the sensitive connections to support regions.In this case, the system may not be bilaterally symmetrical. Thebilateral symmetry, shown for example in FIG. 4, has a number ofadvantages which are described hereinafter.

Referring next to FIG. 8, a system which is similar to that shown inFIG. 4 but which provides a pair of opposed stress relievers 80 and 82in a bilaterally symmetrical system is illustrated. In this case, thebond pad 22 is situated between a first support 83 and a second support84. The provision of dual stress relievers 80 and 82 further improvesthe preferential response of the system about the center portion of thelead as opposed to providing bending response adjacent points ofsupport.

FIG. 9 is a system similar to that shown in FIG. 5A but using a V-shapedstress reliever 90 as opposed to a U-shaped stress reliever 32 or 34.While FIG. 9 shows a system which is not bilaterally symmetrical, as inthe other cases, a bilaterally symmetrical embodiment may be used aswell. Thus, as shown in FIG. 10, a non-bilaterally symmetricalembodiment corresponding to FIG. 4.

A bilaterally symmetrical embodiment may provide a number of importantadvantages in some embodiments. It may provide greater tolerance inmatching the lead to the bond pad 22 during initial positioning. Asexplained above, the portions proximate to the region 30 may also assistin ensuring that even if the lead is displaced from the positionintended, good contact with the bond pad is still obtained.

Bilateral symmetry may also tend to balance the forces applied to thestress relievers. That is, opposite rotations occur which may tend tobalance the effect supplied to the portions of the lead 20 adjacent thebond 22.

Through the use of the anchor 36, shown in FIG. 4, the leads 20 may beplated in position. The anchor 36 may be coupled to a source ofpotential which may be utilized to plate the leads 20 in place. Forexample, it may be desirable to plate the leads with a gold material. Insome cases the leads may be made of one material and then plated withgold material. The gold material may be desirable in some cases inmaking better bonds. The lead 20 may have a core made of a highlyresilient, ductile material since some stressing of the lead may occurduring its downward displacement to make contact to the bond pad 22.Thus, a lead, made of a core material chosen for ductility such asaluminum, may achieve improved contact characteristics by electroplatinggold over the lead for improved contactibility. Examples of othermaterials for forming the lead core include copper, aluminum, platinum,nickel, and alloys and combinations of those materials.

The connection between the tape and the die 21 may be implemented usingwell known, commercially available tape automated bonding techniques.Automatic positioning equipment may be used to facilitate alignmentbetween the bonding pads 22 and the regions 30 of the leads 20.Similarly, conventional techniques may be utilized to couple the leads20 to the outside world, for example through solder balls. While a ballgrid array packaging technique is illustrated, other connectiontechniques may be used in some embodiments.

Among other advantages of embodiments of the present invention is thatit is no longer necessary to provide elaborate techniques forpositioning the lead on the bond pad. For example, in one knowntechnique, it is necessary to displace the lead towards the pad while atthe same time, axially compressing the lead towards it point of support.This provides the bowing orientation shown in FIGS. 6A and 6B. Because adifferent mechanism is utilized for absorbing stresses in embodiments ofthe present invention, it is not necessary to provide this pre-set bow.Eliminating the pre-set bow may simplify the installation of the leads,reducing the likelihood of failure and reducing cost.

Moreover, with embodiments of the present invention, it is not necessaryto break the lead in the process of positioning it. The process ofbreaking the lead may create weakened or strained locations in the leadresulting in subsequent failure. In addition, a process that does notrequire the lead breaking step may be considerably simpler to implement.

Embodiments of the present invention may be installed using tapeautomated bonding techniques at relatively high speed. The tape may beprovided in a continuous form having a plurality of portions havingsufficient leads to couple to one die. Succeeding sections may be cutoff and applied to a series of succeeding dies.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

What is claimed is:
 1. A method of tape bonding integrated circuitscomprising: providing an offset along the length of a cantilevered beamlead to form an offset cantilevered beam lead; and deflecting saidoffset cantilevered beam lead to contact a die.
 2. The method of claim 1wherein said offset cantilevered beam lead is deflected intermediatelyalong its length.
 3. The method of claim 2 including bonding said leadintermediately along its length to said die.
 4. The method of claim 1including causing said offset cantilevered beam lead to rotate about anaxis transverse to the direction that it was deflected.
 5. The method ofclaim 4 including causing the offset cantilevered beam lead to respondto differential thermal expansion by rotating around an axis transverseto the length of said beam lead.
 6. The method of claim 5 includingcausing said offset cantilevered beam lead to absorb compressive andtensile stresses in said offset.
 7. The method of claim 1 includingproviding a pair of cantilevered beam lead portions to either side of aportion of said lead secured to said die.
 8. The method of claim 7including providing an offset in each of said beam lead portions.
 9. Themethod of claim 8 including providing offsets in each beam lead portion,said offsets extending in opposite directions.
 10. The method of claim 7including forming a pair of unsupported portions in said beam lead oneither of side of said contact to said die.